tickler

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that tickles.
• noun A memorandum, book, or file to aid the memory.

from The Century Dictionary.

• noun One who or that which tickles or pleases.
• noun Something which puzzles or perplexes; something difficult to understand or answer; a puzzle.
• noun A narrow difficult passage or strait on the coast of Newfoundland.
• noun A memorandum-book kept to tickle or refresh the memory; specifically, a book used by bankers, showing, in the order of their maturity, notes and debts receivable by the bank. There is usually a tickler for each month of the year.
• noun A small bottle containing about half a pint (of spirits), or just enough to “tickle”; also, a dram of whisky or brandy.
• noun A small weapon carried on the person, as a pistol or a knife.
• noun A strap with which to whip.
• noun A prong' used by coopers to extract bungs from casks.
• noun A large longicorn beetle, Monohammus titillator, with extremely long antennæ: so called from the habit it has (in common with most of the Cerambycidæ) of gently touching now and then the surface on which it walks with the tips of its long antennæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, tickles.
• noun Something puzzling or difficult.
• noun Com. Cant, U. S. A book containing a memorandum of notes and debts arranged in the order of their maturity.
• noun engraving A prong used by coopers to extract bungs from casks.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person who or thing which amuses, tickles, excites.
• noun A reminder.
• noun A latex condom that has additional protrusions, for enhancing the sexual pleasure of the user.
• noun dated Something puzzling or difficult; a conundrum.
• noun US, dated, slang, business A book containing a memorandum of notes and debts arranged in the order of their maturity.
• noun UK, dated A prong used by coopers to extract bungs from casks.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a file of memoranda or notices that remind of things to be done